Ultraviolet (UV) lasers and light-emitting diodes are increasingly being used advantageously with a number of different devices in a number of different fields. UV laser diodes may provide, for example, much higher storage densities on DVD disks. UV light-emitting diodes, for example, may be used to stimulate commercial phosphors to produce solid-state lighting that one day could possibly replace incandescent and fluorescent lamps. These and other such devices typically will require an emitter (e.g., nitride UV emitter) for which a transparent substrate is a vital component.
It is well known to those skilled in the art that if an LED structure is grown on an opaque substrate which is capable of absorbing the emitted light, then the half of the generated light which is emitted into the substrate will be lost by absorption in the substrate. However, if a substrate is transparent, then the LED can be outfitted with a mirror located below the substrate which serves to back-reflect this light, which then again passes through the substrate back up to the top surface, thus doubling the light output. For example, AlN is transparent out to 200 nm, while GaN is only transparent out to 365 nm. Fortunately, AlN and GaN are mutually soluble in all proportions, and thus alloys of the form AlxGa1−xN can be formed which possess absorption edges between 365 and 200 nm in the UV. Thus an AlGaN substrate can be tailored by varying the Al/Ga ratio to be transparent out to any desired wavelength in order not to absorb the emitted UV light from a particular UV-emitting LED device. Such a transparent AlGaN substrate will yield nitride based UV-emitting LEDs with twice the brightness, compared with one grown on an opaque GaN substrate.
Persistent problems in the nitride semiconductor area, however, have hampered production of UV light-emitting devices using such semiconductors. For example, although a sapphire substrate is transparent at all or most wavelengths of interest, it provides a poor lattice match to nitrides. Even though a free-standing GaN layer may solve the lattice mismatch problem, it typically absorbs light below 365 nm. Nitride layers with high aluminum concentrations grown on sapphire tend to crack, and carborundum (SiC) is totally absorbing at wavelengths in the UV wavelength range.
U.S. Pat. No. 5,625,202 to Chai discloses growing nitride compound semiconductor films (e.g., GaN) on various substrate materials described as modified wurtzite structure oxide compounds. These include Lithium Aluminum Oxide, Sodium Aluminum Oxide, Lithium Gallium Oxide, Sodium Gallium Oxide, Lithium Germanium Oxide, Sodium Germanium Oxide, Lithium Silicon Oxide, Silicon Oxide, Lithium Phosphor Oxide, Lithium Arsenic Oxide, Lithium Vanadium Oxide, Lithium Magnesium Germanium Oxide, Lithium Zinc Germanium Oxide, Lithium Cadmium Germanium Oxide, Lithium Magnesium Silicon Oxide, Lithium Zinc Silicon Oxide, Lithium Cadmium Silicon Oxide, Sodium Magnesium Germanium Oxide, Sodium Zinc Germanium Oxide, and Sodium Zinc Silicon Oxide. The GaN layer remains on the growth substrate.
The Chai '202 patent in particular discloses forming, for example, a UV light emitting diode (LED) comprising an n-type GaN layer that is directly on a LiGaO2 substrate. In the context of deposition techniques for forming the Ga1−xAlxN on the substrate, the Chai '202 patent briefly mentions molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD).
U.S. Pat. No. 6,156,581 to Vaudo et al. discloses growing one of a gallium, aluminum, or indium (Ga, Al, In) nitride layer on a substrate for subsequent fabrication using metalorganic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE). Vapor-phase (Ga, Al, In) chloride is reacted with a vapor-phase nitrogenous compound in the presence of a substrate to form (Ga, Al, In) nitride. The thickness of the base layer is described as being on the order of 2 microns and greater, and the defect density may be on the order of 108 cm−2 or lower.
The Vaudo et al. '581 patent provides a laundry list of proposed foreign substrates including sapphire, silicon, silicon carbide, diamond, lithium gallate, lithium aluminate, zinc oxide, spinel, magnesium oxide, ScAlMgO4, gallium arsenide, silicon-on-insulator, carbonized silicon-on-insulator, carbonized silicon-on-silicon, gallium nitride, etc., including conductive as well as insulating and semi-insulating substrates, twist-bonded substrates (i.e., where the substrate of crystalline material is bonded to another single crystal substrate material with a finite angular crystallographic misalignment), and compliant substrates of a type disclosed in U.S. Pat. No. 5,563,428 to Ek et al. The patent further discloses that in some embodiments, the substrate can be removed to leave a free-standing wafer. The patent provides specific growth information, though, only for sapphire.
U.S. Pat. No. 6,252,261 to Usui et al. discloses a method and device for producing large-area single crystalline III–V nitrides on an oxide substrate. The large-area single crystalline III–V nitrides are generally denoted by AlxInyGa1−x−yN, with x and y ranging from 0 to 1, and x+y being greater than or equal to zero and less than or equal to one. The crystalline nitride is expressly described as being grown on a sapphire substrate (Al2O3)
U.S. Pat. No. 6,218,280 B1 to Kryliouk et al. discloses forming a nitrided layer on a lithium gallate substrate, forming a first GaN layer on the nitrided layer by metalorganic chemical vapor deposition (MOCVD), growing a next GaN portion using halide vapor phase epitaxy, and growing a capping GaN layer again using MOCVD. The GaN layers may then be separated from the substrate. The patent lists a number of other proposed substrates in addition to the specifically disclosed lithium gallate. These other substrates include LiAlO2, MgAlScO4, Al2MgO4 and LiNdO2. Unfortunately, the use of MOCVD results in carbon being incorporated into the GaN wafer. This carbon may be undesirable for many applications where pure GaN is desired.
U.S. Pat. No. 6,086,673 to Molnar discloses using hydride vapor phase epitaxy to produce a nitride layer on a substrate. Defined as a growth substrate, the substrate is expressly described as preferably being sapphire.
U.S. Pat. No. 6,146,458 to Hooper et al. discloses forming a group III nitride on a substrate using molecular beam epitaxy (MBE). The representative group III nitrides listed are GaN, InN, and AlN, as well as their alloys. The substrate is described as including LiAlO2.
U.S. Pat. No. 6,874,747 to Redwing et al. and U.S. Pat. No. 5,679,152 to Tischler et al. each discloses green-blue to UV light emitting semiconductor lasers comprising n-type and p-type nitride layers on a substrate. The nitride layers may include AlGaN, while the substrate is expressly described as being silicon, silicon carbide, gallium arsenide, or sapphire. Tischler et al. expressly states that silicon and silicon carbide are preferred.
An article by Naniwae et al. entitled “Growth of Single Crystal GaN substrate Using Hydride Vapor Phase Epitaxy” in Jnl of Crystal Growth, Vol. 99, 1990, pp. 381–384, discloses growth of GaN films on a sapphire substrate. A pretreatment of gallium and HCl without ammonia for 10–20 minutes at 1030° C. is used to pretreat the sapphire surface prior to metalorganic vapor phase epitaxy (MOVPE) of the GaN film. An article titled “Epitaxial Growth and Orientation of GaN on (100) g-LiAlO2” by Hellmen, et al. explains the lattice matching property of LiAlO2.
An article by Xu et al. entitled “γ-LiAlO2 single crystal: a novel substrate for GaN epitaxy” in the Journal of Crystal Growth, Vol. 193, 1998, pp. 127–132, discloses LiAlO2 as a substrate for GaN film growth. The substrates were pretreated with ammonia, and thereafter the GaN film was grown using metalorganic chemical vapor deposition. Another article by Xu et al. entitled “MOCVD Growth of GaN on LiAlO2 Substrates” in Phys. Stat. Sol. (a) Vol. 176 (1999), pp. 589–593 also discloses an LiAlO2 substrate, an ammonia pretreatment, and MOCVD to form the GaN layer. Unfortunately, the MOCVD process may not be sufficiently fast to produce thicker films. In addition, the precursor gas for deposition is trimethylgallium which results in carbon being undesirably incorporated into the GaN layer.
An article by Waltereit et al. entitled “Nitride semiconductors free of electrostatic fields for efficient white light-emitting diodes” in Letters to Nature, Vol. 406, Aug. 24, 2000, pp. 865–868, discloses the epitaxial growth of a thin layer of M-plane GaN on γ-LiAlO2 using plasma-assisted molecular beam epitaxy. The exposed surface of the thin GaN layer may be bonded to another substrate, and the LiAlO2 layer may then be selectively removed to form certain types of higher efficiency devices.
An article also by Waltereit et al. entitled “Growth of M-Plane GaN(1 100) : A Way to Evade Electrical Polarization in Nitrides” in Phys. Stat. Sol. (a) Vol. 180 (2000) pp. 133–138, similarly discloses the formation of an M-plane GaN layer on LiAlO2 substrate. The thin GaN layer (1.5 μm sample) is grown using molecular beam epitaxy at a relatively slow growth rate of 0.5 μm/h. The article reports that M-plane GaN is free of electrical polarization, as compared to more convention C-plane GaN, and that this leads to improved electron-hole wavefunction overlap and therefore improved quantum efficiencies. The M-plane GaN quantum wells have a dramatic improvement in room-temperature quantum efficiency, and the authors surmise that if contributions from competing non-radiative recombination channels are equal for M-plane and C-plane wells, then M-plane GaN opens the way for highly efficient ultraviolet emission.
Despite continuing developments in the area of GaN film growth, what would still be desired is an efficient approach to produce free-standing, high quality, single crystal, AlGaN wafers that are transparent to light in the UV range.